AstroDrizzle Error Affecting Saturated Sources in WFC3/IR Images

A. Fruchter, J. Mack, & W. Hack

Users retrieving WFC3/IR images from the archive after June 7, 2012 and through late September, 2012 may find that bright objectswhich were saturated in one or more reads have total counts which are lower than expected (up to half a magnitude in some cases). Similarly, WFC3/IR data processed offline with AstroDrizzle prior to version 1.0.5 may have this problem. Version 1.0.5 was released on August 13 and is currently only available in IRAFX. For a complete set of release notes, see the DrizzlePac website.The WFC3/IR detector is typically instructed to perform multiple non-destructive reads over the course of an exposure. A bright source that saturates the detector over the total length of the exposure may be faint enough that those pixels are not saturated in the early reads. In this case, the calibration software uses the unsaturated reads to calculate the photon flux and reports the exposure time of the pixel as the total time in the unsaturated reads. Thus a bright star may have its central pixel, as well as some of the surrounding pixels saturated in a number of reads, with pixels further out remaining unsaturated throughout the entire length of the exposure. The central saturated pixels will have shorter exposure times recorded than those around them, with those pixels saturated earliest having the shortest exposure times of all.In the latter stages of AstroDrizzle development, a change was made to use this pixel-based exposure time, rather than the total exposure length when drizzling. This was done to allow users to more accurately compute photometric errors using the output weight map. Unfortunately, the regression tests that were in place at the time did not catch an error that this scheme can introduce. When an image is drizzled, adjacent pixels, one of which may be saturated and the other not, can contribute to the value of the same output pixel. Using pixel-based exposure time weighting (which is employed by the archive), the unsaturated pixel would receive a higher weight since it had a longer exposure time. This will bias the output count value low. We are therefore returning to a flat exposure time weight across the entire image. Users who manually reprocessed saturated WFC3/IR data with AstroDrizzle prior to version 1.0.5 may also encounter this problem when using either exposure time weighting or "inverse variance map" weighting.Again, this error will only affect users attempting photometry (or very precise astrometry) on sources bright enough to have saturated in one or more reads. Therefore many users can ignore this issue. However, if this bug could affect your science, we recommend reprocessing. To determine whether an IR image contains saturated pixels, users may inspect either the [TIME] or [SAMP] extensions of the image, where the exposure time and number of samples will be lower in pixels that were saturated. We will provide an update on the DrizzlePac webpage when the archive is using a version of AstroDrizzle containing this fix. In the future we will continue to look into other approaches for simplifying the creation of accurate pixel-level final exposure maps for WFC3/IR.

UVIS Zeropoints

S. Deustua & J. Mack

The WFC3 zeropoints represent the magnitude corresponding to a (flat-field corrected) count rate of 1 e ⋅ s -1 and are used to convert from instrumental to astrophysical units. The tables provided at http://www.stsci.edu/hst/wfc3/phot_zp_lbn summarize results determined from a new reduction of all SMOV4, Cycle 17, and Cycle 18 observations of GD153, G191B2B, GD71, and P330E (Kalirai and Rajan, 2012, in preparation). Independent calibrations from these four stars agree to within 1% in most filters. The tabulated photometric zeropoint is the average of all the measurements in each filter. This is a substantial improvement over the previous calibration, where a filter-specific solution was not calculated (i.e., a smooth curve was fit across all wavelengths, ignoring small departures). The new data sets were processed with the new AstroDrizzle software and made use of the improved WFC3 flat fields released in December 2011. The new UVIS flats are substantially better than the previous generation, removing large-scale spatial variations and an internal reflection, or 'flare', present in the laboratory flats.

Two sets of tables are provided for both the IR and UVIS filters. The first set gives the zeropoints for an infinite aperture, the second set are the zeropoints corrected for an r=0.4 arcsec aperture. For users who use other apertures, or wish to make their own corrections, aperture corrections and encircled and ensquared energy tables are available at the bottom of the webpage.

These PHOTFLAM values are consistent with the values written to the image header for UVIS1 (sci,2 or ext4). The UVIS2 values computed by SYNPHOT differ from UVIS1 by ~2-3% for filters with pivot wavelength < 300nm and > 700nm. The affected filters are F218W, F225W, FQ232N, FQ243N, F275W, F280N, F300X at the shortest wavelengths and F600LP, FQ727N, FQ750N, F763M, F775W, F814W, F845M, F850LP, FQ889N, FQ906N, FQ924N, FQ937N at the longest wavelengths. The discrepancy in zeropoints for F953N is much larger, ~7.7%. For all other filters, UVIS1 and UVIS2 agree to better than 1%. A single zeropoint should be used for both chips, as given in the tables. We are working to resolve the discrepancy in SYNPHOT.

WFC3/UVIS CTE and Post-flash

L. Dressel, J. Anderson, S. Baggett, & J. MacKenty

Declining Charge Transfer Efficiency (CTE) became apparent in images made with WFC3/UVIS relatively early in its on-orbit lifetime. This is partly due to the fact that WFC3 was installed during solar minimum, when the cosmic ray flux is greatest and radiation damage most rapid, and partly due to the low background often found in UVIS images. The low background has facilitated searches for faint sources, a combination of conditions where CTE losses are largest and most problematic. To mitigate CTE losses, the WFC3 team has developed the use of a post-flash capability to increase the background in UVIS exposures. This capability was announced to the community in July 2012 in the CTE White Paper (MacKenty and Smith), which discusses the current state of CTE for the WFC3/UVIS and ACS/WFC detectors and makes several recommendations, including the use of post-flash, to mitigate CTE losses. Post-flash is now a supported exposure-level option for WFC3/UVIS exposures, both for cycle 20 programs and for programs with unexecuted visits from earlier cycles.

The mechanics, stability, and uniformity of the UVIS post-flash are described in TIR WFC3 2012-01 ("WFC3/UVIS TV3 Post-Flash Results", Baggett and Wheeler), wherein dark exposures taken with post-flash during the final ground-based thermal vacuum testing campaign are analyzed. The optimal level of background to achieve using post-flash in science exposures (12 electrons per pixel) has been determined from a series of exposures of the globular cluster Omega Cen, as discussed in "The Efficacy of Post-Flashing for Mitigating CTE Losses in WFC3/UVIS Images" (Anderson, MacKenty, Baggett, and Noeske). For increasing amounts of background above that level, noise continues to increase but CTE does not continue to improve. The background level (sky plus dark) achieved in 1000 seconds, and the exposure time to reach a level of 10 electrons in that background, is given for most UVIS filters in ISR WFC3 2012-12 ("WFC3/UVIS Sky Backgrounds", Baggett and Anderson) to assist observers in determining whether post-flash would be beneficial, and, if so, what level to select.

Stability of the WFC3 Geometric Distortion

V. Kozhurina-Platais

The stability of WFC3/UVIS and IR geometric distortion is an important matter for accurate alignment of WFC3 images using the STSDAS software AstroDrizzle. It requires accurate distortion correction to combine dithered WFC3 images (UVIS and IR), to rectify the WFC3 images, to enhance the spatial resolution, and to deepen the detection limit. Any significant uncertainty unaccounted for in the geometric distortion correction would be detrimental to the alignment of WFC3 images. The main uncertainties in the geometric distortion are the so-called skew term and potential variations of X and Y scales over time.

For these reasons, it is important to examine and monitor the linear part of WFC3 geometric distortion and forecast its evolution with time. In order to do so, observations of the globular cluster Omega-Cen taken through F606W UVIS and F160W IR filters were used to examine the linear part of the WFC3/UVIS & IR geometric distortion and to search for variations of the astrometric scale with time. As described by Kozhurina-Platais & Petro (WFC3-ISR-2012-03), the WFC3/UVIS and IR geometric distortion appears to be stable and there is no evidence of a secular change on the two-year time scale, with a possible exception of occasional variations of the IR skew, which is crucially dependent on the accuracy of centering techniques used for measuring the X and Y positions of severely under-sampled IR drizzled images. Despite the long term stability, during each interval of orbital target visibility, there is a linear trend of the skew in UVIS and IR X & Y axes at the level of +/-0.05 UVIS and IR pixels (or about 2 and 7 mas at the far edges of UVIS and IR drizzled images), respectively. This temporal dependency of skew variations appears to be related to the focus variations over an HST orbital time scale, known as orbital breathing.

The studies of WFC3/UVIS and IR distortion stability over time will continue in the calibration Cycle 20.

Update to WFC3 Flight Software affecting the UVIS shutter control

M. Sosey, A. Welty, & S. Baggett

The flight software for WFC3 was updated on Thursday, August 16, 2012 in order to change how the shutter on the UVIS detectors behaves when the Take Data Flag (TDF) is down at the start of a science exposure. There have been relatively few documented cases of the TDF down at exposure start over the history WFC3 data, so this should be affecting a small subset of both archived and future data. The main reason for TDF being down is a Guide Star Acquisition which took longer than expected.

The previous functionality was as follows:

whenexptime < 200s: ignore the TDF state and open the shutter for the full exposure

whenexptime > 200s: if the TDF is DOWN at the start of the exposure, never open the shutter and set exptime = 0

The new functionality will be as follows:

whenexptime < 200s: ignore the TDF state and open the shutter for the full exposure

whenexptime > 200s: ignore the TDF state and open the shutter. If the TDF state changes (goes down), then close the shutter for the remainder of the exposure and update the exptime, TDF down time and number keywords appropriately (subtracting the time that the shutter was closed from the commanded exposure time)

This flight software change does not require any updates to the actual data processing steps in CALWF3. However, in order to make the shutter status more obvious to users as well as more accurate than it is currently recorded, CALWF3 will be updated so that the EXPFLAG stored in processed data is a bit more explicit. Currently, EXPFLAG is set to one of three values by generic conversion:

NORMAL: exposure occurred as planned

INTERRUPTED: the TDF went DOWN sometime during the exposure

INDETERMINATE: the TDF was down before the exposure

When associations are processed in WFC3, the header information in the higher level product uses the header from just one of the member datasets. This means the EXPFLAG could contain any one of the three settings and if the user (or archive browser) doesn't look at all the members then there is little warning that there could have been an issue with one of the exposures in the association. The new version of CALWF3 will poll the members for the EXPFLAG and if any one of them is not NORMAL then EXPFLAG in the higher level product header will be set to MIXED. The full EXPFLAG information for all member datasets will be recorded in the history comments of the file for reference. The calibration software update will be implemented in a release of calwf3 planned for this fall.